Knowledge Base
BMU: Using Dual Scanners
FILED UNDER: BMU, Burners, GN ElectronicsSymptom
The BMU (and other flame safeguards) can accept more than one flame scanner input.
All versions of the BMU have dual flame scanner inputs.
Only one scanner is required, a second scanner can be used (based on site conditions, see below).
How the BMU uses dual scanners:
For each scanner, the BMU has a separate contact closure input (Flame / No Flame) and a separate 4-20mA (0-100%) Flame Strength input.
The contact closure inputs are used for the safe/unsafe, run/lockout, decision.
The 4-20mA flame strength signals are NOT used for run/lockout decisions.
The 4-20mA wiring can be disconnected and the burner will function normally, and safely based on the flame scanner contact input signal(s).
The 4-20mA Flame Strength signal is displayed to assist with burner tuning, scanner aiming, troubleshooting, and to determine when scanner cleaning is required.
When dual scanners are installed, if either one of the two flame scanner contact inputs is made, the burner will continue to fire.
If either scanner contact input is made during the Safe Start check, or Purge, the burner will lockout.
When the burner is firing, both scanner contacts must open in order to cause a lockout.
This is the standard dual scanner logic used by almost all BMS manufacturers for a single burner application.
When dual scanners are used, the BMU can trigger an optional warning alarm if either scanner 4-20mA signal drops below a field adjustable threshold.
This warning can be used to let the operator know that one of the scanners needs to be cleaned, or is malfunctioning.
The warning alarm does not cause a lockout, and the alarm level adjustment does not influence the contact closure input that is used for the run/lockout decision.
Typical Dual Scanner Applications (usage depends on individual site conditions):
Increased reliability/up-time:
With a single scanner, if the lens gets dirty, the burner can trip due to a “loss of flame”, even though the flame is actually present.
With dual scanners, the burner will continue to fire based on the clean scanner.
The dirty scanner can be cleaned when convenient.
Depending on scanner aiming/sighting, it may be possible to clean the dirty scanner while the burner is running.
Dual scanners are widely used in manufacturing plants that cannot tolerate down-time and in safety critical chemical/oil/pharmaceutical plants.
Gas-Oil Burners:
Oil fires primarily emit Infrared (IR). Gas fires emit primarily Ultraviolet (UV).
While one scanner can frequently detect both oil and gas, the flame signal for one of the fuels is typically much weaker and this can cause nuisance lockouts due to a marginal flame scanner signal, even though the flame is present and strong. Some oil burners use steam to atomize the oil. The steam blocks much of the UV emitted from an oil fire.
In addition, some pilot flame are long, lazy, and yellow for reliable light-off. These gas pilots sometime emit more IR than UV.
With dual scanners, one can be an IR scanner and one can be a UV scanner.
Low NOx burners:
Low NOX flames with FGR are typically longer, lazier, and less well formed. The flame front location can vary substantially from low to high fire.
For light-off safety, at least one scanner must be aimed such that it “sees” the intersection of the pilot flame and the main flame, typically close to the burner front.
As the firing rate increases a single scanner will remain aimed at the base of the flame, and the flame signal can thus decrease as the firing rate increases and moves away from the burner front.
A second scanner can be aimed away from the pilot and see the flame at higher firing rates, and thus prevent nuisance lockouts.
Low O2 burners:
Low O2 flames are typically longer, and lazier.
A second scanner can be aimed to better see the flame at high fire.
Burners with a larger turn-down:
A second scanner can be aimed to better see the flame at high fire.
Multiple scanners are a long proven, widely used, and safe technique.
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